Multithreading in Java

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Multithreading in Java

• Nelson Padua-Perez
• Bill Pugh
• Department of Computer Science
• University of Maryland, College Park

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Problem

• Multiple tasks for computer
• Draw display images on screen
• Check keyboard mouse input
• Send receive data on network
• Read write files to disk
• Perform useful computation (editor, browser,
  game)
• How does computer do everything at once?
• Multitasking
• Multiprocessing

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Multitasking (Time-Sharing)

• Approach
• Computer does some work on a task
• Computer then quickly switch to next task
• Tasks managed by operating system (scheduler)
• Computer seems to work on tasks concurrently
• Can improve performance by reducing waiting

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Multitasking Can Aid Performance

• Single task
• Two tasks

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Multiprocessing (Multithreading)

• Approach
• Multiple processing units (multiprocessor)
• Computer works on several tasks in parallel
• Performance can be improved

4096 processor Cray X1
32 processor Pentium Xeon
Dual-core AMD Athlon X2
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Perform Multiple Tasks Using

• Process
• Definition executable program loaded in memory
• Has own address space
• Variables data structures (in memory)
• Each process may execute a different program
• Communicate via operating system, files, network
• May contain multiple threads

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Perform Multiple Tasks Using

• Thread
• Definition sequentially executed stream of
  instructions
• Shares address space with other threads
• Has own execution context
• Program counter, call stack (local variables)
• Communicate via shared access to data
• Multiple threads in process execute same program
• Also known as lightweight process

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Motivation for Multithreading

• Captures logical structure of problem
• May have concurrent interacting components
• Can handle each component using separate thread
• Simplifies programming for problem
• Example

Web Server uses threads to handle
Multiple simultaneous web browser requests
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Motivation for Multithreading

• Better utilize hardware resources
• When a thread is delayed, compute other threads
• Given extra hardware, compute threads in parallel
• Reduce overall execution time
• Example

Multiple simultaneous web browser requests
Handled faster by multiple web servers
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Multithreading Overview

• Motivation background
• Threads
• Creating Java threads
• Thread states
• Scheduling
• Synchronization
• Data races
• Locks
• Wait / Notify

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Programming with Threads

• Concurrent programming
• Writing programs divided into independent tasks
• Tasks may be executed in parallel on
  multiprocessors
• Multithreading
• Executing program with multiple threads in
  parallel
• Special form of multiprocessing

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Creating Threads in Java

• You have to specify the work you want the thread
  to do
• Define a class that implements the Runnable
  interface
• public interface Runnable
• public void run()
• Put the work in the run method
• Create an instance of the worker class and create
  a thread to run it
• or hand the worker instance to an executor

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Thread Class

• public class Thread
• public Thread(Runnable R) // Thread ?
  R.run()
• public Thread(Runnable R, String name)
• public void start() // begin thread execution
• ...

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More Thread Class Methods

• public class Thread
• public String getName()
• public void interrupt()
• public boolean isAlive()
• public void join()
• public void setDaemon(boolean on)
• public void setName(String name)
• public void setPriority(int level)
• public static Thread currentThread()
• public static void sleep(long milliseconds)
• public static void yield()

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Creating Threads in Java

• Runnable interface
• Create object implementing Runnable interface
• Pass it to Thread object via Thread constructor
• Example
• public class MyT implements Runnable
• public void run()
• // work for thread
• Thread t new Thread(new MyT()) // create
  thread
• t.start() // begin running thread
• // thread executing in parallel

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Alternative (Not Recommended)

• Directly extend Thread class
• public class MyT extends Thread
• public void run()
• // work for thread
• MyT t new MyT() // create thread
• t.start() // begin running thread

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Why not recommended?

• Not a big problem for getting started
• but a bad habit for industrial strength
  development
• The methods of the worker class and the Thread
  class get all tangled up
• Makes it hard to migrate to Thread Pools and
  other more efficient approaches

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Threads Thread States

• Java thread can be in one of these states
• New thread allocated waiting for start()
• Runnable thread can execute
• Blocked thread waiting for event (I/O, etc.)
• Terminated thread finished
• Transitions between states caused by
• Invoking methods in class Thread
• start(), yield(), sleep()
• Other (external) events
• Scheduler, I/O, returning from run()

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Threads Thread States

• State diagram

new
start
IO complete, sleep expired, join
complete, acquire lock
new
runnable
terminate
blocked
IO, sleep, join, request lock
terminated
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Threads Scheduling

• Scheduler
• Determines which runnable threads to run
• Can be based on thread priority
• Part of OS or Java Virtual Machine (JVM)
• Many computers can run multiple threads
  simultaneously (or nearly so)

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Java Thread Example

• public class ThreadExample implements Runnable
• public void run()
• for (int i 0 i lt 3 i)
• System.out.println(i)
• public static void main(String args)
• new Thread(new ThreadExample()).start()
• new Thread( new ThreadExample()).start()
• System.out.println("Done")

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Java Thread Example Output

• Possible outputs
• 0,1,2,0,1,2,Done // thread 1, thread 2, main()
• 0,1,2,Done,0,1,2 // thread 1, main(), thread 2
• Done,0,1,2,0,1,2 // main(), thread 1, thread 2
• 0,0,1,1,2,Done,2 // main() threads interleaved

main () thread 1, thread 2, println Done
thread 1 println 0, println 1, println 2
thread 2 println 0, println 1, println 2
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Daemon Threads

• Why doesnt the program quit as soon as Done is
  printed?
• Java threads types
• User
• Daemon
• Provide general services
• Typically never terminate
• Call setDaemon() before start()
• Program termination
• If all non-daemon threads terminate, JVM shuts
  down

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Might not see different interleavings

• The threads in that example are too short
• Each started thread will probably complete before
  the next thread starts
• Lets make more threads that run longer

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Data Races

• public class DataRace implements Runnable
• static volatile int x
• public void run()
• for (int i 0 i lt 10000 i)
• x
• x--
• public static void main(String args) throws
  Exception
• Thread threads new Thread100
• for (int i 0 i lt threads.length i)
• threadsi new Thread(new
  DataRace())
• for (int i 0 i lt threads.length i)
• threadsi.start()
• for (int i 0 i lt threads.length i)
• threadsi.join()
• System.out.println(x) // x not always
  0!

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Why volatile

• Well spend more time on volatile later
• But volatile tells the compiler
• other threads might see reads/writes of this
  variable
• dont change/reorder eliminate the reads and
  writes
• An optimizing compiler should, if it sees
• x x--
• replace it with a no-op
• if x isnt volatile

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Thread Scheduling Observations

• Order thread is selected is indeterminate
• Depends on scheduler, timing, chance
• Scheduling is not guaranteed to be fair
• Some schedules/interleavings can cause unexpected
  and bad behaviors
• Synchronization
• can be used to control thread execution order

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Using Synchronization

• public class DataRace implements Runnable
• static volatile int x
• static Object lock new Object()
• public void run()
• for (int i 0 i lt 10000 i)
  synchronized(lock)
• x x--
• public static void main(String args) throws
  Exception
• Thread threads new Thread100
• for (int i 0 i lt threads.length i)
• threadsi new Thread(new
  DataRace())
• for (int i 0 i lt threads.length i)
• threadsi.start()
• for (int i 0 i lt threads.length i)
• threadsi.join()
• System.out.println(x) // x always 0!